An often used commercial system for the separation of air is cryogenic rectification. The separation is driven by elevated feed pressure which is generally attained by compressing feed air in a compressor prior to introduction into a column system. The separation is carried out by passing liquid and vapor in countercurrent contact through the column or columns on vapor liquid contacting elements whereby more volatile component(s) are passed from the liquid to the vapor, and less volatile component(s) are passed from the vapor to the liquid. As the vapor progresses up a column it becomes progressively richer in the more volatile components and as the liquid progresses down a column it becomes progressively richer in the less volatile components. Generally the cryogenic separation is carried out in a main column system comprising at least one column wherein the feed is separated into nitrogen-rich and oxygen-rich components, and in an auxiliary argon column wherein feed from the main column system is separated into argon-richer and oxygen-richer components.
Often it is desired to recover the product gas from the air separation system at an elevated pressure. Generally this is carried out by compressing the product gas to a higher pressure by passage through a compressor. Such a system is effective but is quite costly.
In response to this problem there have been developed air separation processes wherein liquid oxygen is pressurized, such as by pumping or by hydrostatic means, and vaporized against an air stream which is either partially or totally condensed. This markedly reduces the compression costs for the elevated pressure oxygen gas product.
One problem with such systems is that all of the condensed air enters the high pressure column of the air separation plant near the bottom of the column. The condensed air undergoes practically no distillation compared to air entering as a vapor at the bottom of the high pressure column. As a result, nitrogen, which is usually available as liquid nitrogen reflux for operation of the high pressure column and the top portion of the low pressure column when all air enters the high pressure column as a vapor, is not separated from the liquid air. Since the reflux ratio of the high pressure column is fixed by the purity of reflux withdrawn from the top of the column and the number of equilibrium stages present in the column, there is produced less reflux for operation of the top portion of the upper column. Because of this, the recovery of argon will be less than for a comparable process where oxygen is withdrawn as a vapor from the bottom of the low pressure column. Further, if any of the nitrogen separated is recovered as a liquid, the reflux available to the top portion of the upper column will be even less. In fact, it is possible through the production of a sufficient quantity of liquid nitrogen to reduce the quantity of reflux to below a point known as minimum reflux where the L/V ratio in the top portion of the upper column is not sufficient to achieve the desired product purity at any recovery level.
Accordingly it is an object of this invention to provide a system for the separation of feed air by cryogenic rectification wherein elevated pressure oxygen gas may be produced by vaporizing liquid oxygen against condensing feed air and wherein the upper column or lower pressure column of the air separation plant may be operated with an improved liquid to vapor (L/V) ratio to improve the degree of separation in the lower pressure column.